Heat pump system for paper machine dryers



April 26, 1960 E. J. JUSTUS 5 9 HEAT PUMP SYSTEM FOR PAPER MACHINEDRYERS Filed Jan. 2, 1957 J My [04:41? 1/ (/wras' operation of thedryers.

HEAT PUMP SYSTEM FOR PAPER MACHINE DRYERS Edgar .I. Justus, Beloit,Wis., assignor to Beloit Iron Works, Beloit, Wis, a corporation ofWisconsin Application January 2, 1957, Serial No. 632,075

8 Claims. (Cl. 34-86) The instant invention relates to an improved papermachine dryer arrangement, and more particularly, to an improved heatingsystem for paper machine dryers.

In a paper machine dryer system of the type customarily used, there aremounted a plurality of rolls, usually in a pair of tiers, and the wetpaper web is fed into the dryer beneath the roll in the lower tier, thenover a roll in the upper tier thereabove and down beneath the next rollin the lower tier, and so on in a serpentine path through the entiredryer. Each of the rolls is a hollow drum rotatably mounted receivingsteam through a coaxial inlet and each of the rolls is provided with acoaxially aligned condensate removal line. The steam heated rolls causemoisture in the paper web to be evaporated in order to obtain thedesired moisture content in the final paper. In the usual arrangement,the condensate is returned to a boiler and steam is regeneratedtherefrom and returned to the dryer drums or rolls.

The steam entering into the dryer drums is steam of about to 160 poundsper square inch pressure ordinarily, and the moisture driven out of theweb is approximately at the temperature of the boiling point of water(212 F.) at atmospheric pressure.

I In order to protect the operators, prevent overheating of the dryerroom, and control the rate of evaporation of the moisture in the webpassing through the dryers, it is customary to enclose the dryer in ahood which permits controlled flow of incoming air up over the dryerdrums .and which controls the flow of the off-running stream of air andwater vapor coming off the top of the dryer. Heretofore, the customarypractice has been to vent the hood to the atmosphere outside thebuilding.

Theinstant invention involves a unique saving in the In the instantarrangement the condensate returned from the dryer drums is convertedinto steam, but the heat employed for such conversion is taken from theair-vapor mixture ordinarily vented from the hood. Actually, the bulk ofthe heat is obtained by condensation of the vapor carried with the airout of the hood. The dryer drum condensate is converted to steam, usingsuch heat, by subjecting the condensate to vacuum conditions, so as toobtain therefrom steam under subatmospheric pressure. Such steam is thenconverted to superheated steam at a greater (usually, superatmospheric)pressure by passing the same through a compressor or pump. Thecompressor converts the steam to superheated steam at the operatingpressure for the :dryer'drum (which may be about -5 to 160 pounds per.square inch in a typical operation).

Preferably, the heat in the vented air-vapor mixture is recovered bysubject- 'ing this mixture to a water spray which brings the temperatureof the air-vapor mixture substantially below its 'dew point to effectcondensation of most of the vapor.

Patenit' vert the dryer drum condensate (under vacuum) to steam which isthen fed into the compressor suction. In the instant method andapparatus great saving results in the use of mechanical energy (via thecompressor) in combination with the recovery of the heat from the ventedair-vapor mixture.

It is, therefore, an important object of the instant invention toprovide an improved apparatus and method for paper machine dryeroperation.

It is a further object of the instant invention to provide an improvedheat recovery system for paper machine dryers and an improved method ofutilizing heat and mechanical energy in the operation of paper machinedryers.

It is another object of the instant invention to provide an improvedpaper machine dryer system comprising a dryer part having a steam inletand condensate outlet, a hood surrounding the drums having an air inletand an air-vapor outlet, first heat exchange means connected to saidhood outlet and having a cold water inlet and hot water outlet, secondheat exchange means having one pass with an inlet connected to saidfirst means water outlet and a second pass, separated from the first,provided with an inlet connected to said condensate outlet and anoutlet, and a pump having an intake connected to said second pass outletand a discharge connected to said steam inlet.

Still another object of the instant invention is to provide an improvedmethod which comprises condensing a stream of steam against a surfacecontacting a wet paper web to vaporize moisture therein, withdrawingcondensate from said surface, subjecting such condensate tosubatmospheric pressure and heat to vaporize the same, compressing thevaporized condensate to produce superheated steam, directing suchsuperheated steam into said stream of steam, sweeping a stream of airover said web to pick up vaporized moisture therefrom, and withdrawingheat from the air stream carrying the vaporized moisture to heat thecondensate at subatmospheric pressure.

Othere and further objects, features and advantages of the presentinvention will become apparent to those skilled in the art from thefollowing detailed disclosure thereof and the drawing attached heretoand made a part hereof.

defining an air-vapor outlet at 12d (the fiow of air and vapor beingindicated by arrows). Actually, a plurality of dryer drums 11 mounted ina pair of tiers are employed within the hood 12, but for simplicity onlya single dryer drum 11 is here shown. The hood substantially enclosesthe dryer drums 11, 11. A small amount of air may enter the hood (at theweb inlet and outlet) by diffusion against a positive small pressuredifference between the hood interior and ambient atmosphere.

In the instant invention a conduit He carries the vapor mixture (whichmay contain substantially all vapor) from the outlet 12d to a first heatexchange system 13. In the heat exchanger 13 relatively cool water issprayed through a sparger 13a into the air-vapor stream wherein the coolwater is heated (primarily by the heat of vaporization of the vaporwhich is condensed thereby) and hot water flows out a bottom exit 13b.The gas remaining (which may be mostlyair with substantially all of thevapor pickedv up in the hood 12 removed therefrom) continues through anoutlet duct 12f leading away from the heat exchanger 13 and, if desired,this gas may then be vented to the atmosphere through a purge valve 14;or (as will be explained hereinafter) this gas may continue aroundthrough a duct 12g into the suction port 15a of a blower 15 and out thedischarge port 15b thereof back into the hood 12 through a conduit 1211.As will be appreciated, if this gas is released to the atmospherethrough the purge valve 14 at the end of the conduit 12f, thenatmospheric air enters into the hood inlet conduit 12h through a valve16. When the blower 15 is in operation the valves 14 and 16 are closed.An advantage in the use of the blower 15 is that the gas recycled iswarmer than atmospheric air (which would come through the valve 16) anda heat loss involved in heating atmospheric air up to operatingtemperatures in the hood12 is eliminated, at the cost of the operationof the blower 15.

As here shown, the water circulated through the first heat exchanger 13is continuously recycled. The water passes through the sparger 13a, theair-vapor mixture and finally the hot water outlet 13b at the bottom ofthe heat exchanger 13 into the inlet 17a of a'second heat exchanger 17.In contrast to the first heat exchanger 13 which permits actual contactbetween the fluids undergoing the heat exchange, the second heatexchanger 17 is provided with a first or outside pass 17b which receivesthe water from the first exchanger 13 through the inlet 17a and a secondor inside pass which is maintained separate from the first pass' 17bbecause it is enclosed within a bundle of tubes 170 which define thesame. As will be appreciated, other forms of heat exchange structure maybe employed, but the heat exchanger 17 is provided with separated passes17b and 170. The hot water flowing through the outside pass 17b flowsover the tubes of the inside pass 170 collecting in the bottom 17d ofthe heat exchanger 17 from which it is withdrawn through a valve 18 andconduit 19 into a reservoir 20. The water in the reservoir 20 has lostsome of its heat which was removed by the second pass 170 and this wateris then returned to the sparger 13a through conduit 21 and pump 22.Since this cycle will continuously gain in water content because of thecontinuous removal of vapor from the air-vapor stream, an overflow valve23 is connected to the reservoir for main,- taining the total amount ofwater in the system substantially constant;

Referring now to the steam-condensate system associated with the dryerdrum 11, itwill be seen that the condensate is withdrawn from the drum11 through a condensate outlet line 11b and returned to a reservoir orhot well 24. From the hot well 24 water vapor and/ or condensate isdrawn (or optionally pumped by a pump not shown) through a conduit25 andvalve 26 into the inlet side of the second pass 170 of the heatexchanger 17 at 17c, by a compressor 27 having its intake 27a connectedto the second pass outlet 17,. The compressor 27 maintainssubatmospheric pressure within the second pass 17c, so thatcompletevolatilization of the condensate therein takes place to formsteam at subatmospheric pres sure. The compressor 27 then converts thesteam at subatmospheric pressure to superheated steam atsuperatmospheric pressure coming from the compressor discharge 27b.

The compressor discharge 27b feeds to the steam inlet 11a for the drum11, but preferably the superheated steam at superatmospheric pressure isconverted to saturated steam at superatmospheric pressure at a mixingstation 28 whereat condensate and/or water vapor is introduced from thehot well 24 through a conduit 29 by a pump 30. By the addition of apredetermined amount of condensate to the mixing station 28 saturatedsteam isobtained'at the steam inlet 11a for the -drum 11.

per square inch gauge. predetermined amount of condensate is not sentthrough Considering a specific set of operating conditions, it will beseen that a substantial economic advantage is obtained in the practiceof the instant invention. At a production rate of 233 pounds per minutepaper, the paper entering the dryers is 33.9% dry and the paper leavingthe dryers is 91.3% dry. The water removed per pound of paper is thus1.693 pound per pound of paper produced. The water evaporated at thisproduction rate is thus 395 pounds per minute. The heat load on thedryers may thus be computed as follows:

B.t.u./min. (a) Amount required to heat paper and residual Water from F.to 212 F. 10,848 (b) Amount required to heat and evaporate 395 lbs. ofwater from 85 to 212 F. 433,749 (0) Heat loss from dryer and hood byradiation and convection 6,710 (d) Miscpfriction losses 276 Total451,582

In operating the present system, the water comes through the sparger 13aat 190 F. and exits from the exchanger 13 at 200 F., wherein this waterhas gained 387,200 B.t.u./min. (through condensation of 391 pounds perminute of vapor at 212 F.). The inside pass is maintained at 9.3 poundsper square inch (absolute) so 427.5 lbs. per minute of condensate isvaporized by the energy gained in the exchanger 13. The vaporizedcondensate is then converted to 40 pounds per square inch (ga.) steaminto which 71.2 pounds per minute of condensate is fogged at the mixingstation 28 to obtain saturated steam.

It is apparent from the foregoing that the bulk of the 'heat load on thedryer is taken up by the amount required to evaporate the 395 pounds ofwater per minute. In the specific embodiment here involved, the hood 12is not sealed but only small openings are permitted resulting in a lossof about 1% of the vapor produced within the hood 12. The remainder ofthis vapor, which is about 391 pounds per minute is then fed into theheat exchanger 13 at about 212 F. Relatively cool water at about F.'isfed into the exchanger 13 through the spargers 13a and the resultingwater coming out the exit 13b of the heat exchanger 13 is 200 F. In theheat exchanger 13 (which is a cooling tower embodiment here' employed)and bulk of the heat of vaporization imparted to the system by the drum11 is recovered. The amount of vapor carried out of the cooling tower 13is a negligible amount, even though the air leaving the cooling tower 13is substantially saturated (at about 190 F.). In the instant operation,the amount of air passing over the dryer 11 is only about 0.183 poundsper minute and this can carry away only a negligible amount of the 391pounds of water vapor in the conduit 12 The amount ofheat actuallyrecovered in the second heat exchanger 17 is in excess of 375,000B.t.u./minute (actually it is 387,200 B.t.u./min.) and the powerrequired to operate the compressor 27 is 76,400 B.t.u./minute. Thecompressor power is thus the only energy added to the system. i v

In the instant operation, the specific operating conditionsinvolve'maintaining a pressure in the inside pass 17c of the heatexchanger 17 of 9.3 pounds per square inch absolute (at which waterboils at 190 F.) and the compressor 27 discharges superheated steam at40' pounds As previously mentioned, a small the heat exchanger 17 butinstead is fogged into the superheated steam in the mixing chamber 28 toconvert the same to saturated steam. It will thus be seen that, assumingmechanical and thermal energy to be substantially equal in cost (on thebasis of B.t.u./min.) the saving in the instant operation is 83%. 1

If the operation is changed so as to produce 100 pounds 6 per squareinch steam at the discharge of the compressor 27, the compressor powerrequired is 126,000 B.t.u'./minute and the saving is thus 72%. In theembodiments just described, the amount of air employed in the system isso small that there is no need to operate the blower 15, and there isconsequently no mechanical energy input at this point. If, however,greater amounts of air are used, then the mechanical energy input atthecompressor may be balanced against the heat energy required to heatfresh incoming air from room temperature to the dryer temperature, alsotaking into consideration the heat retained in the system in the form ofWater vapor carried by the air'through the cooling tower 13.

Since the hood pressure is maintained most advantageously at atmosphericpressure, the air-vapor mixture taken therefrom well always be less than212 F. It is thus necessary to operate the inside pass 170 of the heatexchanger 17 at subatmospheric pressure sufiicient to permitboiling orwater therein at a temperature below 212 F. There is, thus a maximumpermitted pressure within the inside pass 17c which is required in orderto obtain evaporation of the condensate therein, but the minimumoperating pressure is primarily a practical consideration. The dischargepressure of the compressor 27 is, of course, greater than the intakepressure (and preferably atmospheric pressure) which is the pressure atwhich the dryer drum 11 is maintained. This pressure may also varywithin practical limits. The primary consideration here is that ofincreasing temperature of the steam coming out of the second pass orinside pass 17c of the heat exchanger 17 to the extent necessary to makeup for heat not recovered from the system to the operation of the firstheat exchanger 13.

It will be understood that modifications and variations maybe efi'ectedwithout departing from the spirit and scope of the novel concepts ofthis invention.

I claim as my invention:

1. In a paper machine dryer system, a dryer drum having a steam inletand condensate outlet, a hood surrounding the drum having an air inletand an air-vapor outlet, first heat exchange means connected to saidhood outlet and having a cold water inlet and hot water outlet, secondheat exchange means having one pass with an inlet connected to saidfirst means water outlet and a second pass, separated from the first,provided with an inlet connected to said condensate outlet and anoutlet, and a pump having an intake connected to said second pass outletand a discharge connected to said steam inlet, said pump convertingwater vapor at low pressure in said second pass to vapor at higherpressure in said steam inlet, thereby adding heat to the system.

2. In a paper machine dryer system, a dryer drum having a steam inletand condensate outlet, a reservoir connected to said condensate outlet,a hood surrounding the drum having an air inlet and an air-vapor outlet,first heat exchange means connected to said hood outlet and having acold water inlet and hot water outlet, second heat exchange means havingone pass with an inlet connected to said first means water outlet and asecond pass, separated from the first, provided with an inlet connectedto said reservoir and an outlet, and a pump having an intake connectedto said second pass outlet and a discharge connected to said steaminlet, said pump maintaining water vapor at subatmospheric pressure insaid pass and maintaining water vapor at superatmospheric pressure insaid steam inlet by compressing water vapor passing through the pump soas to add heat to the system.

3. In a paper machine dryer system, a dryer drum having a steam inletand condensate outlet, a reservoir connected to said condensate outlet,a hood surrounding the drum having an air inlet and an air-vapor outlet,first heat exchange means connected to said hood outlet and having acold water inlet and hot water outlet, sec-. ond heat exchange meanshavingone pass with an inlet connected to said first means water outletand a second pass, separated from the first, provided with an inletconnected to said reservoir and an outlet, a pump having an intakeconnected to said second pass outlet and a discharge connected to saidsteam inlet, and a conduit interconnecting said pump discharge and saidreservoir, said pump converting water vapor at low pressure in saidsecond pass to vapor at higher pressure insaid steam inlet, therebyadding heat to the system.

4."1n a paper machine dryer system, a dryer drum having a steam inletand condensate outlet, a hood surrounding the drum having an air inletand an air-vapor outlet, first heat exchange means connected to saidhood outlet and having a cold water inlet and hot water outlet, secondheat exchange means having one pass with an inlet connected to saidfirst means waterv outlet and having'a water outlet connected tothe coldwater inlet of said first means and a second pass, separated from thefirst,- provided with an inlet connected to said condensate outlet andan outlet, and a pump having an intake connected to said second passoutlet and a discharge connected to said steam inlet, said pumpmaintaining water vapor at subatmospheric pressure in said pass andmaintaining water vapor at superatmospheric pressure in said steam inletby compressing water vapor passing through the pump so as to add heat tothe system.

5. In a paper machine dryer system, a dryer drum having a steam inletand condensate outlet, a hood surrounding the drum having an air inletand an air-vapor outlet, first heat exchange means connected to saidhood outlet and having a cold water inlet spraying water into anair-vapor stream passing therethrough and a hot water outlet, a blowerreceiving air passing through said first heat exchange means anddischarging air into the hood air inlet, second heat exchange meanshaving one said second pass outlet and a discharge connected to saidsteam inlet, said pump maintaining water vapor at subatmosphericpressure in said pass and maintaining water vapor at superatmosphericpressure in said steam inlet by compressing water vapor passing throughthe pump so as to add heat to the system.

6. In a paper machine dryer system, a dryer drum having a steam inletand condensate outlet, a reservoir connected to said condensate outlet,a hood surrounding the drum having an air inlet and an air-vapor outlet,first heat exchange means connected to said hood outlet and having acold water inlet spraying water into an airvapor stream passingtherethrough and a hot water outlet, a blower receiving air passingthrough said first heat exchange means and discharging air into the hoodair inlet, second heat exchange means having one pass with an inletconnected to said first means water outlet and a second pass, separatedfrom the first, provided with an inlet connected to said reservoir andan outlet, a pump having an intake connected to said second pass outletand a discharge connected to said steam inlet, and a conduitinterconnecting said pump discharge and said reservoir, said pumpmaintaining water vapor at subatmospheric pressure in said pass andmaintaining water vapor at superatmospheric pressure in said steam inletby compressing water vapor passing through the pump so as to add heat tothe system.

7. In a paper machine dryer system, a dryer drum having a steam inletand condensate outlet, a reservoir connected to said condensate outlet,a hood surrounding the drum having an air inlet and an air-vapor outlet,first heat exchange means connected to said hood outlet and having acold water inlet spraying Water into an air-vapor stream passingtherethrough and a hot water outlet, 9.

blower receiving air passing through said first heat exchange means anddischarging air into the hood air inlet, second heat exchange meanshaving one pass with an inlet connected to said first means water outletand having a water outlet connected to the cold water inlet of saidfirst means and a second pass, separated from the first, provided withan inlet connected to said reservoir and an outlet and a pump having anintake connected to said second pass outlet and a discharge connected tosaid steam inlet, and a conduit interconnecting said pump discharge andsaid reservoir, said pump maintaining water vapor at subatmosphericpressure in said pass and main: taining Water vapor at superatmosphericpressure in said steam inlet by compressing Water vapor passing throughthe pump so as to add heat to the system.

8. In a paper machine dryer system, a dryer drum having a steam inletand condensate outlet, a reservoir connected to said condensate outlet,a hood surrounding the drum having an air inlet and an air-vapor outlet,first heat exchange means connected to said hood outlet and havinga coldWater inlet and hot water outlet, second heat exchange means having onepass with an inlet con! nected to said first means water outlet andhaving a Water outlet connected to the cold water inlet of said firstmeans and a second pass, separated from the first, provided with aninlet connected to said reservoir and an outlet, a pump having an intakeconnected to said second pass outlet and a discharge con'..ected to saidsteam inlet, and a conduit interconnecting said pump discharge and saidreservoir, said pump maintaining water vapor at subatmospheric pressurein said pass and maintaining water vapor at superatmospheric pressure insaid steam inlet by compressing water vapor passing through the pump soas to add heat to the system.

2,718,710 Spooner Sept. 27, 1955' may

